1. Field of the Invention
The present invention relates to a phosphor and a process for the preparation of the same, and more particularly, to a divalent europium activated alkaline earth metal complex halide phosphor and a process for the preparation of the same.
2. Description of the Prior Art
There is well known a divalent europium activated alkaline earth metal fluorohalide phosphor (M.sup.II FX:Eu.sup.2+, in which M.sup.II is at least one alkaline earth metal selected from the group consisting of Ba, Sr and Ca; and X is a halogen other than fluorine), as a divalent europium activated alkaline earth metal halide phosphor. The phosphor gives emission (spontaneous emission) in the near ultraviolet region when exposed to a radiation such as X-rays. The phosphor also gives emission (stimulated emission) in the near ultraviolet region when excited with an electromagnetic wave such as visible light or infrared rays after exposure to a radiation such as X-rays, that is, the phosphor is a stimulable phosphor.
The present inventor discovered a novel divalent europium activated alkaline earth metal halide phosphor having the following formula:
M.sup.II X.sub.2.aM.sup.II X'.sub.2 :xEu.sup.2+ in which M.sup.II is at least one alkaline earth metal selected from the group consisting of Ba, Sr and Ca; each of X and X' is at least one halogen selected from the group consisting of Cl, Br and I, and X.noteq.X'; and a and x are numbers satisfying the conditions of 0.1.ltoreq.a.ltoreq.10.0 and 0&lt;x .ltoreq.0.2, respectively, and applied for a patent with respect to said phosphor (Japanese Patent Provisional Publication No. 60(1985)-84381, corresponding to U.S. patent application No. 660,987 and European Patent Application No. 84112417.5).
The novel divalent europium activated alkaline earth metal halide phosphor has been confirmed to have a crystal structure different from that of the aforementioned M.sup.II FX:Eu.sup.2+ phosphor on the basis of the X-ray diffraction patterns as described in the above application. This phosphor gives spontaneous emission (peak wavelength: approx. 405 nm) in the near ultraviolet to blue region upon exposure to a radiation such as X-rays, ultraviolet rays or cathode rays, and also gives stimulated emission in the near ultraviolet to blue region when excited with an electromagnetic wave having a wavelength within the region of 450-1000 nm after exposure to a radiation such as X-rays, ultraviolet rays or cathode rays. Accordingly, the phosphor is very useful for a radiographic intensifying screen employed in the conventional radiography and for a radiation image storage panel employed in the radiation image recording and reproducing method utilizing a stimulable phosphor.
It is desired that the luminance of spontaneous emision of this useful phosphor is as high as possible.
SUMMARY OF THE INVENTION
The present invention provides a novel divalent europium activated alkaline earth metal halide phosphor which is improved in the luminance of spontaneous emission and a process for the preparation of the same.
The present inventors have studied on the above-mentioned novel divalent europium activated alkaline earth metal halide phosphor, and as a result, they have found that a phosphor further containing a specific amount of alkali metal halide shows spontaneous emission of high luminance, to accomplish the invention.
The phosphor of the invention is a divalent europium activated alkaline earth metal complex halide phosphor having the formula (I): EQU M.sup.II X.sub.2.aM.sup.II X'.sub.2.bM.sup.I X":xEu.sup.2+ (I)
in which M.sup.II is at least one alkaline earth metal selected from the group consisting of Ba, Sr and Ca; M.sup.I is at least one alkali metal selected from the group consisting of Li and Na; each of X and X' is at least one halogen selected from the group consisting of Cl, Br and I, and X.noteq.X'; X" is at least one halogen selected from the group consisting of F, Cl, Br and I; and a, b and x re numbers satisfying the conditions of 0.1.ltoreq.a.ltoreq.10.0, 0&lt;b.ltoreq.2.times.10.sup.-1 and 0&lt;a.ltoreq.0.2, respectively.
The process for the preparation of the phosphor having the formula (I) of the invention comprises:
mixing starting materials for the phosphor in a stoichiometric ratio corresponding to the formula (II): EQU m.sup.II X.sub.2.aM.sup.II X'.sub.2.bM.sup.I X":xEu (II)
in which
in which M.sup.II, M.sup.II X', X", a, b and x have the same meanings as defined above; and
firing the obtained mixture at a temperature within the range of 400.degree.-1300.degree. C. in an inert atmosphere or a weak reducing atmosphere.
In the invention, the luminance of spontaneous emission given when exciting a phosphor with a radiation such as X-rays is enhanced by incorporating the specific amount of alkali metal halide (sodium halide and/or lithium halide) into the novel divalent europium activated alkaline earth metal halide phosphor.